Since many types of machinery require several rates of travel along an axis during a machine cycle, this is a common machine design problem usually sought to be solved. For example, in metal cutting machine tools, it is frequently necessary to provide a rapid traverse or approach rate, where the tool comes from a clearance or home position, to a point near the workpiece where the tool then drops to a cutting feed rate. In a screw feed system for this application, it would be desirable to employ a screw which has a variable lead to obtain a variable axial feed velocity output resulting from a constant angular velocity input.
Molding machines, such as plastic injection molding machines, often have a continuous stroke cycle where a mold platen is moved along an axis with respect to a fixed platen, during a first stroke portion, in order to close a mold after a part has been extracted; a second stroke portion is used to drive the mold platens tightly against one another to withstand the injection pressures. The second stroke portion would usually involve a higher force than the first stroke portion of movement. In a screw feed system for this application, it would be desirable to employ a screw which produces a variable mechanical advantage with a nut system, based on constant input conditions.
It is well-known to provide two different threads on a common screw shaft in both "compound" and "differential" screw systems. The textbook, Kinematics of Machines, second edition, Prentice-Hall, Inc., defines these systems. A compound screw consists of two screws in series arranged so that the resultant motion produced is the sum of the individual motions. A differential screw consists of two screws in series arranged so that the resultant motion produced is the difference of the individual motions. In other words, with reference to a single thread system, a compound screw might be considered a coarse positioning device and a differential screw might be considered a fine positioning device.
One prior art device utilizes a compound screw to provide rapid positioning of a machine part, then converts to use only one of the two threads, for finer feed. U.S. Pat. No. 4,797,086, T. Adachi, Jan. 10, 1989 teaches an apparatus for closing a mold, where a movable mold platen is driven through part of its travel, by a compound screw. Two spaced nuts are used, with the first one fixed to the machine base and the second one mounted on the movable platen, the platen nut being selectively fixed, or freely rotatable, according to the state of an electric clutch. A feed screw has different threads at each end, of opposite hand helixes. When the electric clutch is energized, fixing the second nut, and a torque is transmitted to the screw through a spline, the screw moves through the first nut, and the second nut and platen move along the screw. Thus, the platen sees the addition of the two screw thread leads. The screw has an integrally-fixed mechanical clutch member between the end threads, and a mating mechanical clutch member is integrally-affixed to the second nut. At the precise time the mechanical clutch elements engage, the electric clutch must be deenergized; the now free-wheeling second nut then simply rotates with the screw while the first nut and mating screw thread continue propelling the platen in conventional single thread fashion.
Several difficulties are inherent in the '086 patent. The screw must be mounted to linearly advance while rotating. Lead errors in both screw threads are seen throughout the rapid feed stroke. An energizeable clutch must be provided, with precise energy control at the point where the mechanical clutch engages and disengages, in order to maintain a smooth, continuous motion. These difficulties are obviated by a feed system which uses two different thread portions on a screw where a nut system can smoothly translate from one thread to the other while the screw is rotating.